The miniaturization of electronic components has resulted in a profusion of small, lightweight, expensive electrical appliances. Television sets, stereo equipment, and home computers are just a few of the appliances constituting a class of highly desirable, easily stolen, and easily resold merchandise. Historically, the motel industry has been the primary market for electrical appliance anti-theft devices. This market has now expanded to include a broad spectrum of residential and commercial applications, and will continue to grow in the foreseeable future.
The present invention is a theft protection device for electrical appliances. There has been a great deal of effort extended in this area. I have encountered over sixty related patents which I have grouped into six classifications: The first group uses modifications of the plug receptacle to detect an unplug condition. Usually a special box is connected between the appliance and the regular outlet. The removal of the appliance plug closes an alarm circuit. U.S. Pat. No. 3,484,775, issued to W. D. Cline on Dec. 16, 1969, is an example. This approach is subject to a number of problems. Most notably, a thief could cut the cord prior to stealing the device. Alternately, he cold pry loose the box from the wall and take it with him, without unplugging the unit. Because the alarm unit is exposed, the designer bears the full brunt of protecting it from destruction. A fair degree of protection can be obtained by embedding the device in the building structure, but only at significant cost.
The second approach involves setting up special wiring networks linking the device with a central monitoring area. U.S. Pat. No. 3,766,540, issued to Schapfer, et al., on Oct. 16, 1973, utilizes such a configuration. These networks require extensive wiring and generally cost more than the appliances they are designed to protect. It is important to attempt to make use of the standard wiring already installed or in use.
The third and fourth categories are AC power and motion detectors. These are often used in combination because, separately, they are especially prone to false alarms--the former due to power failures, the latter due to innocent vibrations. Roger S. Lent was issued U.S. Pat. No. 4,284,983 for such a combination Aug. 18, 1981. The combination is basically effective. It can be circumvented, however, by using an extension cord to keep the appliance powered while moving it. A common hundred foot extension cord would allow a thief to transport appliances from a building to a waiting truck with only minor inconvenience.
The fifth method, proposed in U.S. Pat. No. 3,423,747 issued to H. C. Hogencamp Jan. 21, 1969, combines a power sensing circuit with a loop utilizing the ground connection within the household wiring. The alarm connects to two separate grounding points and monitors the continuity of the resulting loop. The intent is to provide a second alarm criterion to distinguish power failures and thefts. The resulting combination becomes no more effective than the ground loop alone. The loop can be readily simulated by shorting the alarm leads together. The removal of the device will then be falsely interpreted as a power failure.
Finally, E. M. Tellerman, et al., in U.S. Pat. No. 3,425,050, issued Jan. 28, 1969, uses the continuity of a different loop. The ground and neutral or cold wire of standard household wiring are shorted together at the power distribution panel. Tellerman verifies the continuity of this loop as a method of determining if the appliance is plugged in. Unfortunately, this loop is subject to the same shorting constraints as the Hogencamp loop. A screwdriver held across the plug terminals will disable the alarm.
The device described in this patent also relies on the neutral to ground loop of the Tellerman device. Instead of checking for continuity, however, an actual measurement is made of the loop length using time domain reflectometry. A pulse is transmitted down the transmission line formed by these two wires and is reflected off the short at the distribution panel. The time required for the voltage pulse to drop to its steady-state zero voltage level is a function of the distance it must travel to reach the distribution panel. This time can be stored as a code known only to the alarm itself and compared to subsequent pulses. If the appliance is unplugged, the voltage pulse never encounters a short circuit and, therefore, never drops to a zero voltage level. The resulting pulse duration becomes infinite. Furthermore, any attempt to simulate the distribution panel with a new short will be detected by the change in loop length.